The present invention relates to an optical printer head and a printer using the same, more particularly to an optical printer head provided with a liquid crystal shutter array and a printer using such a head.
A device is conventional which includes a liquid crystal shutter array and a fluorescent lamp for illuminating it. However, this type of device involves a drawback in that a longer period of time is required from energization of the lamp to the irradiation of sufficient quantity of light, and therefore, a longer waiting period is needed before the necessary intensity of the information light is reached in the optical printer head. Additionally, the fluorescent lamps produce quite a large amount of heat, which leads to the possibility of an adverse affect to the liquid crystal shutter array assembly. Furthermore, the bulkiness of the fluorescent lamp and its accessories hinders reduction of device size. Another type of device is known as not having the above drawbacks and is shown in FIG. 1. This device employs as the light source an LED (light emitting diode) array 12 comprising a plurality of LED elements arranged in the direction perpendicular to the sheet of the drawing. The light emitted by the LED array 12 is condensed by a cylindrical lens 13 on a light receiving surface of an LCS (liquid crystal shutter) array assembly 15 comprising an LCS array which includes a plurality of LCS elements arranged in the direction perpendicular to the sheet of the drawing of FIG. 1. The cylindrical lens 13 does not have light condensing power in the longitudinal direction of the LED array 12 and the LCS array assembly 15, that is, the direction perpendicular to the sheet of the drawing. It has the light condensing power only in a plane perpendicular to the length of the arrays. The LCS array assembly 15 emits light from its light emitting surface, which is received by an imaging element array 16 comprising a plurality of short focus imaging elements having a small diameter, for example small diameter lenses or refractive index dispersing type imaging elements of small diameter which is available, for example, from Nippon Itagaras Kabushiki Kaisha, trade name SELFOC, which are arranged in the longitudinal direction of LED or LCS array, namely the direction perpendicular to the sheet of the drawing. The imaging element array 16 focuses the light on a photosensitive number which is being moved.
In this type of a printer head, the light emitted by the LED array 12, that is, the light source, is only partly used effectively. This is firstly because the liquid crystal shutter array assembly transmits only a part of linearly polarized component. This stems from the fundamental property of a liquid crystal shutter array assembly, so that it is unavoidable. The second reason is that while the cylindrical lens 13 is effective to condense the light in one dimension, it does not condense the light in the other dimension, i.e. in the longitudinal direction of the cylindrical lens 13. In other words, the light from the LED array 12 diverges in the longitudinal direction of the cylindrical lens 13 with the result that there exists a light component which can not be received by an aperture of the array 16. In addition, the cylindrical lens 13 itself absorbs a part of the light. Therefore, the significance of the condensing lens 13 is, in effect, smaller than expected. For those reasons, the exposure amount of the photosensitive member tends to be insufficient. To obviate this, an increased amount of current is supplied to the LED 12. As a result, the service life of the LED element is decreased, and the printer head is easily damaged.
Additionally, since the width of the picture element (individual LCS) array of the LCS array assembly 15 is so small that it is difficult to position each of the elements in such a manner that the light from the LED array 12 is condensed just in alignment with the width of the cylindrical lens 13. If, for example, the light from the LED array 12 impinges on the LCS array assembly 15 inclinedly, the light quantity for each of the picture elements is not uniform, resulting in non-uniform image density in the image formed on the photosensitive member. Further, the structure wherein the light from the LED array 12 is projected through the cylindrical lens 13, requires the space and optical path for the cylindrical lens 13. Therefore, the printer head is necessarily bulky, which of course makes a printer bulky, too.